ounts of
organic acids, as malic acid found in apples, citric in lemons, and
tartaric in grapes. These give characteristic taste to foods, but have
no direct nutritive value. They do not yield heat and energy as do
starch, fat, and protein; they are, however, useful for imparting flavor
and palatability, and it is believed they promote to some extent the
digestion of foods with which they are combined by encouraging the
secretion of the digestive fluids. Many fruits and vegetables owe their
dietetic value to the organic acids which they contain. In plants they
are usually in chemical combination with the minerals, forming compounds
as salts, or with the organic compounds, producing materials as acid
proteins. In the plant economy they take an essential part in promoting
growth and aiding the plant to secure by osmotic action its mineral food
from the soil. Organic acids are found to some extent in animal foods,
as the various lactic acids of meat and milk. They are also formed in
food materials as the result of ferment action. When seeds germinate,
small amounts of carbohydrates are converted into organic acids. In
general the organic acids are not to be considered as nutrients, but as
food adjuncts, increasing palatability and promoting digestion.

15. Essential Oils.--Essential or volatile oils differ from fats, or
fixed oils, in chemical composition and physical properties.[6] The
essential oils are readily volatilized, leaving no permanent residue,
while the fixed fats are practically non-volatile. Various essential
oils are present in small amounts in nearly all vegetable food
materials, and the characteristic flavor of many fruits is due to them.
It is these compounds which are used for flavoring purposes, as
discussed in Chapter IV. The amount in a food material is very small,
usually only a few hundredths of a per cent. The essential oils have no
direct food value, but indirectly, like the organic acids, they assist
in promoting favorable digestive action, and are also valuable because
they impart a pleasant taste. Through poor methods of cooking and
preparation, the essential oils are readily lost from some foods.

16. Mixed Compounds.--Food materials frequently contain
compounds which do not naturally fall into the five groups
mentioned,--carbohydrates, pectose substances, fats, organic acids, and
essential oils. The amount of such compounds is small, and they are
classed as miscellaneous or mixed non-nitroge